1. Field of the Invention
The present invention relates a super twisted nematic type liquid crystal display device.
2. Description of the Related Arts
Conventionally, a spacer is used to maintain at a constant value the gap between a pair of upper and lower electrode substrate which hold liquid crystal therebetween in a liquid crystal display device using glass substrates.
As such spacer, cylindrical glass fibers, plastic beads or spherical SiO.sub.2 grains are used. The diameter of the glass fibers can be controlled with high accuracy depending on the manufacturing method thereof. More particularly, the standard deviation of the fiber diameters is on=0.1 .mu.m or less. Therefore, if a liquid crystal display device of a plastic film substrate type is made using glass fibers as the spacer, the resulting cell gap is of high accuracy.
Glass fibers are distributed over and adhered to the whole of at least one of opposing surfaces of the upper and lower electrode substrates constituting a package.
However, the glass fibers are cylindrical, and has a length density of about 20 .mu.m, so that if they are dispersed at a high density, for example, of 30 fibers/mm.sup.2 or more, many fibers would overlap and even dispersion of the fibers would be difficult. Since the glass fibers are made of an inorganic material, and has a significant hardness, they may damage transparent conductive films on the plastic film substrates to thereby causing disconnections.
Specially, the rate increases at which disconnections occur at liquid crystal display devices having thin electrodes to perform graphic display and thus the glass fibers have been regarded as not suitable for the plastic substrates.
Although the spherical grains are not overlapped through high density dispersion to thereby damage on the transparent conductive films, the accuracy of the grain diameters is not high compared to that of glass fibers. For example, the accuracy of the plastic bead diameter is low (their standard deviation .sigma. is 0.4 .mu.m), and the cell gap is uneven. In this connection, 0.4&gt;.sigma.&gt;0.25 for the plastic beads and 0.2&gt;.sigma. for SiO.sub.2. The shape of the beads is spherical, so that the beads as the spacers are likely to move within the cell, regions where there are fewer beads appear at reliability test and uneven color distribution occurs.
Recently, a display system has been proposed and put already to practical use which operates in a mode referred to as SBE (super twisted birefringence effect) or STN (super twisted numatic) having a twisted angle of 180.degree. or more.
STN type liquid crystal displays have a display mode applying a double refraction of liquid crystal to a display system, STN type liquid crystal displays have a rising characteristics of liquid crystal shaper than that of conventional twisted nematic type liquid crystal displays (TN LCDs), it is possible for STN type LCD to have a panel with a large display capacity.
STN type liquid crystal displays are recently used in word processor or personal computer as a display unit. They can be provided with 640.times.480 dots (1/480 Duty) at maximum.
According to this type, the color changes by a small change .DELTA.n.d (where .DELTA.n=ne-no, ne is the refractive index of liquid molecules along their longer axes, no is the refractive index of the liquid molecules along their shorter axes, and d is the cell gap), so that if there are variations in the gap of the cell, uneven color distribution is produced. It is clear from the results of simulations and experiments that an allowance of .DELTA.n.d in which no uneven color distribution cannot be recognized by the naked eye is about .+-.0.005 .mu.m or less. The value of .DELTA.n varies depending on liquid crystal to be used and if it is assumed to be about 0.1, a change in d is .+-.0.05 .mu.m or less.
Japanese Patent Laying-Open (KOKAI) No. 63-96634 has disclosed a LCD in which the quantity of spacer dispersion is arranged to be 120 to 250 grains/mm.sup.2. The most suitable value of a gap can obtained by making the quantity of the dispersion the most suitable (in the case where a glass panel is used). However, the quantity of the dispersion causes the scattering of the values of the gap in the case of a PF-LCD. Therefore, fine irregular color appears.
Japanese Patent Laying-Open (KOKAI) No. 60-260022 has disclosed a PF-LCD using spherical beads and in which the quantity of the dispersion is arranged to be 50 to 100 grains/mm.sup.2. In the case of a TN type PF-LCD, since the influence of change in a cell gap upon the display quality is not significant, the display quality is not changed significantly by the quantity of the dispersion. Therefore, it is significant for the quality of the spray to be 50 to 100 grains/mm.sup.2. However, in the case of a STN type, displayed colors are changed due to change in .DELTA.n.d (where .DELTA.n: the birefringent anisotropy, d: the cell gap). Therefore, the accuracy in the cell gap is changed depending upon the quantity of the dispersion, causing the irregular color to easily take place. If the dispersion quantity is 50 to 100 grains/mm.sup.2, fine irregular color appears.
Japanese Patent Laying-Open (KOKAI) No. 62-86331 has disclosed a device arranged in such a manner that the diameters of grains are 1 to 10 .mu.m and the standard deviation value of the distribution of the diameters of grains is 0.3 .mu.m or less.
On the other hand, a spherical spacer displays a larger standard distribution value than that of glass fiber. For example, the Micropearl SP manufactured by Sekisui Chemical Co., Ltd. displays a standard deviation value of about 0.4 .mu.m. Since substrate are aligned along the gap material in the case of the PF-LCD, the value of the standard deviation completely corresponds to the irregular gap.
We found facts as a result of the studies that substantially no irregular color can be observed in the case of a cell of 8 .mu.m or more if the standard deviation value is 0.3 .mu.m or less, and that visual irregular color appears when the standard deviation value is 0.3 .mu.m in the case where the cell gap value 6 .mu.m to 7 .mu.m.